Have `restart` event include server version, server feature level.

[chrisbobbe]

As discussed here.

E.g., like

{
  "type": "restart",
  "zulip_version": "4.0-dev-3688-g17b739c3f3",
  "zulip_feature_level": 55
}

or whatever format makes the most sense / is most consistent with the current server API. (The zulip_version and zulip_feature_level names are based on the /server_settings response.)

[zulipbot]

Hello @zulip/server-api members, this issue was labeled with the "area: api" label, so you may want to check it out!

[m-e-l-u-h-a-n]

@zulipbot claim